Components and catalysts for the polymerization of olefins

ABSTRACT

The present invention relates to a solid catalyst component for the polymerization of olefins CH 2 ═CHR in which R is hydrogen or a hydrocarbon radical with 1–12 carbon atoms, comprising Mg, Ti, halogen and an electron donor selected from maleates of a particular formula. Said catalyst components when used in the polymerization of olefins, and in particular of propylene, are capable to give polymers in high yields and with high isotactic index expressed in terms of high xylene insolubility.

The present invention relates to catalyst components for thepolymerization of olefins, to the catalyst obtained therefrom and to theuse of said catalysts in the polymerization of olefins CH₂═CHR in whichR is hydrogen or a hydrocarbyl radical with 1–12 carbon atoms. Inparticular the present invention relates to catalyst components,suitable for the stereospecific polymerization of olefins, comprisingTi, Mg, halogen and an electron donor compound selected from esters ofspecifically substituted maleic acids (substituted maleates). Saidcatalyst components when used in the polymerization of olefins, and inparticular of propylene, are capable to give polymers in high yields andwith good isotactic index expressed in terms of high xyleneinsolubility.

Non-substituted maleates and certain substituted maleates are known inthe art and their use as electron donor compounds in the preparation ofsupported Ziegler-Natta catalyst components has already been disclosed.

EP-A-45977 discloses the use of non-substituted maleates as internaldonors in catalyst components for the polymerization of olefins. Theresults obtained are poor both in terms of activity andstereospecificity. In U.S. Pat. No. 5,436,213 esters of maleic orfumaric acids substituted with a C1–C20 hydrocarbon group aregenerically mentioned. The specific disclosure is only directed to2-methyl substituted maleates and in particular to diethyl2-methylmaleate, diisobutyl 2-methylmaleate and didecyl 2-methylmaleate.The said specific maleates show only minor improvements with respect tothe non-substituted maleates. As a whole however, the behaviour of thecatalysts containing these donors is not satisfactory in particular interms of activity.

The Japanese patent application 58(1983)-138708 discloses a process forthe polymerization of olefins carried out in the presence of a catalystone component of which contains magnesium, titanium, halogen and anelectron donor that can also be an ester between a straight-chainalcohol and a substituted or unsubstituted maleic acid. The bestperforming catalysts, according to the said disclosure, would be thosecontaining an ester of a C1–C4 alkyl monosubstituted maleic acid.However, the polymerization results reported in the said applicationshow that also with the preferred donors the catalysts have performancesthat are not particularly attractive in terms of activity andstereospecificity. Considering what is disclosed in the art it wouldappear that Z-N supported catalyst components containing esters ofmaleic acids as internal donors would not be satisfactory in thepolymerization of olefins and in particular of propylene.

It has been therefore very surprising to discover that certain specificsubstituted maleates, when used as internal donors, can give catalystcomponents showing a balance of properties in terms of activity andstereospecificity that renders them particularly suitable for thepolymerization of olefins. Said catalysts in fact show activities and/orstereospecificity dramatically improved over the catalyst componentscontaining the maleates of the prior art as internal donors.

Accordingly, the present invention regards a solid catalyst componentfor the polymerization of olefins CH₂═CHR in which R is hydrogen or ahydrocarbon radical with 1–12 carbon atoms, comprising Mg, Ti, halogenand an electron donor selected from maleates of formula (I):

wherein R′ is a C1–C20 hydrocarbon group optionally containingheteroatoms, R₁ is a C1–C20 hydrocarbon group optionally containingheteroatoms, and R₂ is H or a C1–C20 hydrocarbon group optionallycontaining heteroatoms, with the proviso that when R₂ is H, R₁ isisobutyl or a C5–C20 hydrocarbon group.

When R₂ is H, R₁ is preferably a primary alkyl group having from 5 to 10carbon atoms or a cycloalkyl group. When both R₁ and R₂ are differentfrom H they are preferably selected from C1–C20 alkyl groups.

The R′ groups are preferably primary alkyl, arylalkyl or alkylarylgroups having from 2 to 10 carbon atoms. More preferably they areprimary branched alkyl groups having from 2 to 8 carbon atoms. Examplesof suitable R′ groups are methyl, ethyl, n-propyl, n-butyl, isobutyl,neopentyl, 2-ethylhexyl and trifluoropropyl.

Specific examples of suitable maleates of formula (I) are: Diethyl2-isobutylmaleate, Diisobutyl 2-isobutylmaleate, Di-n-butyl2-isobutylmaleate, Bis(trifluoropropyl) 2-isobutylmaleate, Diethyl2-n-pentylmaleate, Diisobutyl 2-n-pentylmaleate, Diethyl2-cyclohexylmaleate, Diisobutyl 2-cyclohexylmaleate, Di-n-butyl2-cyclohexylmaleate, Diethyl 2-n-decylmaleate, Diisobutyl2-n-decylmaleate, Diethyl 2-cyclopentylmaleate, Diisobutyl2-cyclopentylmaleate, Dimethyl 2-cyclopentylmaleate, Diethyl2-n-decylmaleate Diethyl 2-cycloheptylmaleate, Diethyl 2-benzylmaleate,Diisobutyl 2-benzylmaleate Diethyl 2-cyclohexylmethylmaleate Diethyl2-(2-ethylhexyl)maleate Diethyl 2-(1,3-dimethylbutyl)maleate Diethyl2-(2-pentyl)maleate Diethyl 2-isopentylmaleate Diethyl2-neopentylmaleate, Diethyl 2-(3-pentyl)maleate Diethyl2-(cyano-ethyl)maleate Diethyl 2-(3,3,3-trifluoropropyl)maleate Diethyl2-(3-amino-propyl)maleate Diethyl2-(2,2,2-trifluoro-1-methylethyl)maleate, Diethyl 2,3-dimethylmaleate,Diethyl 2,3-diisobutylmaleate, Diisobutyl 2,3-diisobutylmaleate,Di-n-butyl 2,3-diisobutylmaleate, Bis(trifluoropropyl)2,3-diisobutylmaleate, Diethyl 2,3-di-n-pentylmaleate, Diisobutyl2,3-di-n-pentylmaleate, Diethyl 2,3-dicyclohexylmaleate, Diisobutyl2,3-dicyclohexylmaleate, Di-n-butyl 2,3-dicyclohexylmaleate, Diethyl2,3-di-n-decylmaleate, Diisobutyl 2,3-di-n-decylmaleate, Diethyl2,3-dicyclopentylmaleate, Diisobutyl 2,3-dicyclopentylmaleate, Dimethyl2,3-dicyclopentylmaleate, Diethyl 2,3-dicycloheptylmaleate Diethyl2,3-disecbutylmaleate Diethyl 2,3-dibenzylmaleate, Diisobutyl2,3-dibenzylmaleate Diethyl 2,3-dicyclohexylmethylmaleate Diethyl2,3-bis(2-ethylhexyl)maleate Diethyl 2,3-bis(1,3-dimethylbutyl)maleateDiethyl 2,3-bis(2-pentyl)maleate Diethyl 2,3-diisopentylmaleate Diethyl2,3-dineopentylmaleate, Diethyl 2,3-bis(3-pentyl)maleate, Diethyl2,3-bis(cyano-ethyl)maleate Diethyl2,3-bis(3,3,3-trifluoropropyl)maleate Diethyl2,3-bis(3-amino-propyl)maleate Diethyl2,3-bis(2,2,2-trifluoro-1-methylethyl)maleate, Diethyl2-isobutyl-3-methylmaleate, Diisobutyl 2-isobutyl-3-methylmaleate,Di-n-butyl 2-isobutyl-3-methylmaleate, Diethyl2-n-pentyl-3-ethylmaleate, Diisobutyl 2-n-pentyl-3-n-butylmaleate,Diethyl 2-cyclohexyl-3-propylmaleate, Diisobutyl2-cyclohexyl-3-isopropylmaleate, Di-n-butyl2-cyclohexyl-3-secbutylmaleate.

As explained above, the catalyst components of the invention comprise,in addition to the above electron donors, Ti, Mg and halogen. Inparticular, the catalyst components comprise a titanium compound, havingat least a Ti-halogen bond and the above mentioned electron donorcompound supported on a Mg halide. The magnesium halide is preferablyMgCl₂ in active form which is widely known from the patent literature asa support for Ziegler-Natta catalysts. Patents U.S. Pat. No. 4,298,718and U.S. Pat. No. 4,495,338 were the first to describe the use of thesecompounds in Ziegler-Natta catalysis. It is known from these patentsthat the magnesium dihalides in active form used as support orco-support in components of catalysts for the polymerization of olefinsare characterized by X-ray spectra in which the most intense diffractionline that appears in the spectrum of the non-active halide is diminishedin intensity and is replaced by a halo whose maximum intensity isdisplaced towards lower angles relative to that of the more intenseline.

The preferred titanium compounds used in the catalyst component of thepresent invention are those containing at least one Ti-halogen bond.Preferably TiCl₄, TiCl₃ and Ti-haloalcoholates of formulaTi(OR)_(n-y)X_(y), where n is the valence of titanium, X is halogen andy is a number between 1 and n, are used.

The preparation of the solid catalyst component can be carried outaccording to several methods. According to one of these methods, themagnesium dichloride in an anhydrous state and the maleates are milledtogether under conditions in which activation of the magnesiumdichloride occurs. The so obtained product can be treated one or moretimes with an excess of TiCl₄ at a temperature between 80 and 135° C.This treatment is followed by washings with hydrocarbon solvents untilchloride ions disappeared. According to a further method, the productobtained by co-milling the magnesium-chloride in an anhydrous state, thetitanium compound and the maleate is treated with halogenatedhydrocarbons such as 1,2-dichloroethane, chlorobenzene, dichloromethaneetc. The treatment is carried out for a time between 1 and 4 hours andat temperature of from 40° C. to the boiling point of the halogenatedhydrocarbon. The product obtained is then generally washed with inerthydrocarbon solvents such as hexane.

According to another method, magnesium dichloride is preactivatedaccording to well known methods and then treated with an excess of TiCl₄at a temperature of about 80 to 135° C. which contains, in solution, amaleate of formula (I). The treatment with TiCl₄ is repeated and thesolid is washed with hexane in order to eliminate any non-reacted TiCl₄.

A further method comprises the reaction between magnesium alcoholates orchloroalcoholates (in particular chloroalcoholates prepared according toU.S. Pat. No. 4,220,554) and an excess of TiCl₄ comprising the maleateof formula (I) in solution at a temperature of about 80 to 120° C.

According to a preferred method, the solid catalyst component can beprepared by reacting a titanium compound of formula Ti(OR)_(n-y)X_(y),where n is the valence of titanium and y is a number between 1 and n,preferably TiCl₄, with a magnesium chloride deriving from an adduct offormula MgCl₂.pROH, where p is a number between 0.1 and 6, preferablyfrom 2 to 3.5, and R is a hydrocarbon radical having 1–18 carbon atoms.The adduct can be suitably prepared in spherical form by mixing alcoholand magnesium chloride in the presence of an inert hydrocarbonimmiscible with the adduct, operating under stirring conditions at themelting temperature of the adduct (100–130° C.). Then, the emulsion isquickly quenched, thereby causing the solidification of the adduct inform of spherical particles. Examples of spherical adducts preparedaccording to this procedure are described in U.S. Pat. No. 4,399,054 andU.S. Pat. No. 4,469,648. The so obtained adduct can be directly reactedwith the Ti compound or it can be previously subjected to thermalcontrolled dealcoholation (80–130° C.) so as to obtain an adduct inwhich the number of moles of alcohol is generally lower than 3preferably between 0.1 and 2.5. The reaction with the Ti compound can becarried out by suspending the adduct (dealcoholated or as such) in coldTiCl₄ (generally 0° C.); the mixture is heated up to 80–130° C. and keptat this temperature for 0.5–2 hours. The treatment with TiCl₄ can becarried out one or more times. The maleate can be added during thetreatment with TiCl₄. The treatment with the electron donor compound canbe repeated one or more times.

The preparation of catalyst components in spherical form is describedfor example in European Patent Applications EP-A-395083, EP-A-553805,EP-A-553806, EPA-601525 and WO98/44009.

The solid catalyst components obtained according to the above methodshow a surface area (by B.E.T. method) generally between 20 and 500 m²/gand preferably between 50 and 400 m²/g, and a total porosity (by B.E.T.method) higher than 0.2 cm³/g preferably between 0.2 and 0.6 cm³/g. Theporosity (Hg method) due to pores with radius up to 10.000 Å generallyranges from 0.3 to 1.5 cm³/g, preferably from 0.45 to 1 cm³/g.

A further method to prepare the solid catalyst component of theinvention comprises halogenating magnesium dihydrocarbyloxide compounds,such as magnesium dialkoxide or diaryloxide, with solution of TiCl₄ inaromatic hydrocarbon (such as toluene, xylene etc.) at temperaturesbetween 80 and 130° C. The treatment with TiCl₄ in aromatic hydrocarbonsolution can be repeated one or more times, and the maleate is addedduring one or more of these treatments.

In any of these preparation methods the desired maleate can be added assuch or, in an alternative way, it can be obtained in situ by using anappropriate precursor capable to be transformed in the desired electrondonor compound by means, for example, of known chemical reactions suchas esterification, transesterification etc. Generally, the maleate isused in molar ratio with respect to the MgCl₂ of from 0.01 to 1preferably from 0.05 to 0.5.

The solid catalyst components according to the present invention areconverted into catalysts for the polymerization of olefins by reactingthem with organoalumlinum compounds according to known methods.

In particular, it is an object of the present invention a catalyst forthe polymerization of olefins CH₂═CHR, in which R is hydrogen or ahydrocarbyl radical with 1–12 carbon atoms, comprising the product ofthe reaction between:

-   (a) a solid catalyst component as described above;-   (b) an alkylaluminum compound and, optionally,-   (c) one or more electron-donor compounds (external donor).

The alkyl-Al compound (b) is preferably selected from the trialkylaluminum compounds such as for example triethylaluminum,triisobutylaluminum, tri-n-butylaluminum, tri-n-hexylaluminum,tri-n-octylaluminum. It is also possible to use mixtures oftiialkylaluminum's with alkylaluminum halides, alkylaluminum hydrides oralkylaluminum sesquichlorides such as AlEt₂Cl and Al₂Et₃Cl₃.

The external donor (c) can be of the same type or it can be differentfrom the maleates of formula (I). Suitable external electron-donorcompounds include silicon compounds, ethers, esters such as ethyl4-ethoxybenzoate, amines, heterocyclic compounds and particularly2,2,6,6-tetramethyl piperidine, ketones and the 1,3-diethers of thegeneral formula (II):

wherein R^(I), R^(II), R^(III), R^(IV), R^(V) and R^(VI) equal ordifferent to each other, are hydrogen or hydrocarbon radicals havingfrom 1 to 18 carbon atoms, and R^(VII) and R^(VIII), equal or differentfrom each other, have the same meaning of R^(I)–R^(VI) except that theycannot be hydrogen; one or more of the R^(I)–R^(VIII) groups can belinked to form a cycle. Particularly preferred are the 1,3-diethers inwhich R^(VII) and R^(VIII) are selected from C₁–C₄ alkyl radicals.

Another class of preferred external donor compounds is that of siliconcompounds of formula R_(a) ⁵R_(b) ⁶Si(OR⁷)_(c), where a and b areinteger from 0 to 2, c is an integer from 1 to 3 and the sum (a+b+c) is4; R⁵, R⁶, and R⁷, are alkyl, cycloalkyl or aryl radicals with 1–18carbon atoms optionally containing heteroatoms. Particularly preferredare the silicon compounds in which a is 1, b is 1, c is 2, at least oneof R⁵ and R⁶ is selected from branched alkyl, cycloalkyl or aryl groupswith 3–10 carbon atoms optionally containing heteroatoms and R⁷ is aC₁–C₁₀ alkyl group, in particular methyl. Examples of such preferredsilicon compounds are methylcyclohexyldimethoxysilane,diisopropyldimethoxysilane, diphenyldimethoxysilane,methyl-t-butyldimethoxysilane, dicyclopentyldimethoxysilane,2-ethylpiperidinyl-2-t-butyldimethoxysilane and1,1,1,trifluoropropyl-2-ethylpiperidinyl-dimethoxysilane. Moreover, arealso preferred the silicon compounds in which a is 0, c is 3, R⁶ is abranched alkyl or cycloalkyl group, optionally containing heteroatoms,and R⁷ is methyl. Examples of such preferred silicon compounds arecyclohexyltrimethoxysilane, t-butyltrimethoxysilane andthexyltrimethoxysilane.

The electron donor compound (c) is used in such an amount to give amolar ratio between the organoaluminum compound and said electron donorcompound (c) of from 0.1 to 500, preferably from 1 to 300 and morepreferably from 3 to 100. As previously indicated, when used in the(co)polymerization of olefins, and in particular of propylene, thecatalysts of the invention allow to obtain, with high yields, polymershaving a high isotactic index (expressed by high xylene insolubilityX.I.), thus showing an excellent balance of properties. This isparticularly surprising in view of the fact that, as it can be seen fromthe comparative examples herebelow reported, the use as internalelectron donors of the maleates of the prior art gives worse results interm of yields and/or xylene insolubility.

Therefore, it constitutes a further object of the present invention aprocess for the (co)polymerization of olefins CH₂═CHR, in which R ishydrogen or a hydrocarbyl radical with 1–12 carbon atoms, carried out inthe presence of a catalyst as described above.

Said polymerization process can be carried out according to knowntechniques for example slurry polymerization using as diluent an inerthydrocarbon solvent, or bulk polymerization using the liquid monomer(for example propylene) as a reaction medium. Moreover, it is possiblecarrying out the polymerization process in gas-phase operating in one ormore fluidized or mechanically agitated bed reactors.

The polymerization is generally carried out at temperature of from 20 to120° C., preferably of from 40 to 80° C. When the polymerization iscarried out in gas-phase the operating pressure is generally between 0.5and 10 MPa, preferably between 1 and 5 MPa. In the bulk polymerizationthe operating pressure is generally between 1 and 6 MPa preferablybetween 1.5 and 4 MPa. Hydrogen or other compounds capable to act aschain transfer agents can be used to control the molecular weight ofpolymer.

The following examples are given in order to better illustrate theinvention without limiting it.

Characterizations

Preparation of Maleates

The monosubstituted maleates according to formula (I) used in thepresent invention, can be prepared, for example, by reaction of diethylacetylenedicarboxylate with the corresponding alkylmagnesium chloride inthe presence of a Copper (I) complex.

As an example, the synthesis of diethyl 2-isobutyl maleate is reportedherebelow.

A mechanically stirred suspension of copper(I) bromide-dimethylsulfidecomplex (20.4 g, 99.0 mmol) in 500 mL of THF was cooled to −40° C. andtreated dropwise with 2.0 M solution of i-butylmagnesium chloride in THF(49.5 mL, 99.0 mmol) under an atmosphere of dry nitrogen. After stirringat −40° C. for 2 h, the reaction mixture was cooled to −78° C. and thentreated dropwise with a solution of diethyl acetylenedicarboxylate (13.2mL, 82.5 mmol) in 160 mL of THF. Upon completion of the addition, themixture was stirred at −78° C. for 1 h, quenched with saturated NH₄Claq., then allowed to slowly warm up to room temperature, and stirred atthis temperature for additional 30 min. The organic phase was separatedand the water phase was thoroughly extracted with ether. The combinedorganic extracts were washed with saturated NH₄Cl aq., then with brine,dried over Na₂SO₄, and distilled in vacuum to give 14.8 g (79% yield, bp74–75° C./1 mm Hg) of diethyl 2-1-butylmaleate as a colorless oil.

Propylene Polymerization: General Procedure

In a 4 liter autoclave, purged with nitrogen flow at 70° C. for one our,75 ml of anhydrous hexane containing 800 mg of AlEt₃, 79.8 mg ofdicyclopentyldimethoxysilane and 10 mg of solid catalyst component wereintroduced in propylene flow at 30° C. The autoclave was closed. 1.5 Nlof hydrogen were added and then, under stirring, 1.2 Kg of liquidpropylene were fed. The temperature was raised to 70° C. in five minutesand the polymerization was carried out at this temperature for twohours. The non-reacted propylene was removed, the polymer was recoveredand dried at 70° C. under vacuum for three hours and, then, weighed andfractionated with o-xylene to determine the amount of the xyleneinsoluble (X.I.) fraction at 25° C.

Determination of X.I.

2.5 g of polymer were dissolved in 250 ml of o-xylene under stirring at135° C. for 30 minutes, then the solution was cooled to 25° C. and after30 minutes the insoluble polymer was filtered. The resulting solutionwas evaporated in nitrogen flow and the residue was dried and weighed todetermine the percentage of soluble polymer and then, by difference thexylene insoluble fraction (%).

EXAMPLES Examples 1–5 and Comparative Examples 1–3

Preparation of Solid Catalyst Components.

Into a 500 ml four-necked round flask, purged with nitrogen, 250 ml ofTiCl₄ were introduced at 0° C. Then, were added under stirring 10.0 g ofmicrospheroidal MgCl₂.2.8C₂H₅OH (prepared according to the methoddescribed in ex.2 of U.S. Pat. No. 4,399,054 but operating at 3,000 rpminstead of 10,000) and an amount of maleate such as to give, withrespect to Mg, a molar ratio of 6. The temperature was raised to 100° C.and maintained for 120 min. Then, the stirring was discontinued, thesolid product was allowed to settle and the supernatant liquid wassiphoned off.

250 ml of fresh TiCl₄ were added. The mixture was reacted at 120° C. for60 min and, then, the supernatant liquid was siphoned off. The solid waswashed six times with anhydrous hexane (6×100 ml) at 60° C. Finally, thesolid was dried under vacuum and analyzed. The type and amount ofmaleate (wt %) and the amount of Ti (wt %) contained in the solidcatalyst component are reported in table 1. Polymerization results arereported in table 2.

TABLE 1 Maleate Ti Example Type Wt % Wt % 1 Diethyl 2-isobutylmaleate 83.2 2 Diethyl 2-n-pentylmaleate 10 2.8 3 Diethyl 2-cyclohexylmaleate 8.73.4 4 Diethyl 2-n-decylmaleate 6 3.7 5 Diethyl 2-cyclopentylmaleate 7.53.8 Comp. 1 Diethyl maleate 10.4 5.4 Comp. 2 Diethyl 2-methylmaleate 8.72.6 Comp. 3 Din-butyl maleate 9.4 3.5

TABLE 2 Activity I.I. Example (Kg/g) (%) 1 45 97.2 2 41 97.2 3 54 97.4 443 97 5 38 96.5 Comp. 1 16 93.9 Comp. 2 26 96.6 Comp. 3 26 96.4

1. A solid catalyst component for the polymerization of olefins CH₂═CHR,in which R is hydrogen or a hydrocarbyl radical with 1–12 carbon atoms,comprising Mg, Ti, halogen and an electron donor selected from maleatesof formula (I):

wherein R′ is a C1–C20 hydrocarbon group, R₁ is isobutyl or a C5–C20hydrocarbon group, and R₂ is H.
 2. The solid catalyst component of claim1 wherein R₁ is isobutyl, a primary alkyl group having from 5 to 10carbon atoms or a cycloalkyl group.
 3. The solid catalyst componentaccording to claim 1 in which the R′ groups are primary alkyl, arylalkylor alkylaryl groups having from 2 to 10 carbon atoms.
 4. The solidcatalyst component according to claim 1 comprising a titanium compoundhaving at least a Ti-halogen bond and the maleate supported on Mgdichloride.
 5. The solid catalyst component according to claim 4 inwhich the titanium compound is TiCl₄ or TiCl₃.
 6. The solid catalystcomponent according to claim 1 having a spherical form, a surface area(by B.E.T. method) between 20 and 500 m²/g and a total porosity (byB.E.T. method) higher than 0.2 cm³/g.
 7. A catalyst for thepolymerization of olefins CH₂═CHR, in which R is hydrogen or ahydrocarbyl radical with 1–12 carbon atoms, comprising the productobtained by contacting: (a) a solid catalyst component comprising Mg,Ti, halogen and an electron donor selected from maleates of formula (I):

 wherein R′ is a C1–C20 hydrocarbon group, R₁ is isobutyl or a C5–C20hydrocarbon group, and R₂ is H; (b) an alkylaluminum compound and,optionally, (c) at least one electron-donor compound (external donor).8. The catalyst according to claim 7 in which the alkylaluminum compound(b) is a trialkyl aluminum compound.
 9. The catalyst according to claim8 in which the trialkyl aluminum compound is selected from the groupconsisting of triethylaluminum, triisobutylaluminum,tri-n-butylaluminum, tri-n-hexylaluminum and tri-n-octylaluminum. 10.The catalyst according to claim 7 in which the external donor (c) isselected from the 1,3-diethers of the general formula (II):

wherein R^(I), R^(II), R^(III), R^(IV), R^(V) and R^(VI) equal ordifferent to each other, are hydrogen or hydrocarbon radicals havingfrom 1 to 18 carbon atoms, and R^(VII) and R^(VIII), equal or differentfrom each other, are hydrocarbon radicals having from 1 to 18 carbonatoms; one or more of the R^(I)–R^(VIII) groups can be linked to form acycle.
 11. The catalyst according to claim 7 in which the external donor(c) is a silicon compound of formula R_(a) ⁵R_(b) ⁶Si(OR⁷)_(c), where aand b are integers from 0 to 2, c is an integer from 1 to 4 and the sum(a+b+c) is 4; R⁵, R⁶ and R⁷ are alkyl, cycloalkyl or aryl radicals with1–18 carbon atoms optionally containing heteroatoms.
 12. The catalystaccording to claim 11 in which a is 1, b is 1 and c is
 2. 13. Thecatalyst according to claim 12 in which at least one of R⁵ and R⁶ arebranched alkyl, cycloalkyl or aryl groups with 3–10 carbon atomsoptionally containing heteroatoms and R⁷ is a C₁–C₁₀ alkyl group. 14.The catalyst according to claim 11 in which a is 0, c is 3 and R⁶ is abranched alkyl or cycloalkyl group and R⁷ is methyl.
 15. The catalystaccording to claim 12 or 14 in which the silicon compound is selectedfrom the group consisting of methylcyclohexyldimethoxysilane,diphenyldimethoxysilane, methyl-t-butyldimethoxysilane,dicyclopentyldimethoxysilane cyclohexyltrimethoxysilane,t-butyltrimethoxysilane, thexyltrimethoxysilane,2-ethylpiperidinyl-2-t-butyldimethoxysilane and1,1,1,trifluoropropyl-2-ethylpiperidinyl-dimethoxysilane.
 16. Thecatalyst according to claim 13 wherein R⁷ is methyl.
 17. A process forthe (co)polymerization of olefins CH₂═CHR, in which R is hydrogen or ahydrocarbyl radical with 1–12 carbon atoms, carried out in the presenceof a catalyst comprising the product obtained by contacting: (a) a solidcatalyst component comprising Mg, Ti, halogen and an electron donorselected from maleates of formula (I):

 wherein R′ is a C1–C20 hydrocarbon group, R₁ is isobutyl or a C5–C20hydrocarbon group, and R₂ is H; (b) an alkylaluminum compound and,optionally; (c) at least one electron-donor compound (external donor).